1. CELL and Developmental Biology Graduate Program, University of British Columbia (UBC), 2350 Health Sciences Mall, Vancouver, B.C, V6T 1Z3 Canada
2. Department of Zoology, University of British Columbia (UBC), 2350 Health Sciences Mall, Vancouver, B.C, V6T 1Z3 Canada
3. Department of Microbiology and Immunology, Life Sciences Institute, University of British Columbia (UBC), 2350 Health Sciences Mall, Vancouver, B.C, V6T 1Z3 Canada
4. Fred Hutchinson Cancer Research Center, 1100 Fairview Ave. N. Seattle, WA 98109-1024 USA